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Retaining Wall Calculator: How Many Blocks Do I Need?
The Formula
Wall Length (ft) x Wall Height (ft) / Block Face Area (sq ft) = Number of Blocks
A standard segmental retaining wall block (like Allan Block or Versa-Lok) has a face roughly 18” wide x 6” tall. Face area: 0.75 sq ft.
A 40-foot wall, 3 feet tall:
40 x 3 / 0.75 = 160 blocks
Add 5-10% waste for cuts at corners and curves: 176 blocks.
Block Sizes Vary
Not all retaining wall blocks are the same size. Common face dimensions:
- Large format (Redi-Rock, Recon): 48” x 18” face. One block covers 6 sq ft.
- Standard (Allan Block, Belgard): 18” x 6” face. 0.75 sq ft.
- Small format (tumbled garden wall): 12” x 4” face. 0.33 sq ft.
The block you pick changes the count dramatically. That same 40 x 3 wall: 160 standard blocks, 20 large-format blocks, or 360 small garden blocks. Price per block also varies from $2 for small tumbled to $15 for standard to $200+ for large format. Check your local supplier’s inventory before committing to a design — some brands aren’t stocked in every region.
Cap Pieces
Every finished retaining wall needs a cap course. Caps are sold separately and typically match the block width.
Cap count: Wall Length / Cap Width
For a 40-foot wall with 18” caps: 40 / 1.5 = 27 cap pieces. Buy 28.
Caps glue down with construction adhesive. Two tubes of landscape block adhesive per 40 feet of wall is typical.
Base Course
The base course sits in a trench below grade. This is the most important part of the wall. A bad base creates a crooked wall within two years.
Trench width: block depth + 12 inches behind for gravel. Trench depth: 6 inches of compacted gravel base, with the first course of block buried.
A standard block is 12 inches deep. So the trench is 24 inches wide (12” block + 12” gravel behind) and about 12 inches deep (6” gravel + 6” block height, with the block top near grade level).
Base gravel for our 40-foot wall: 40 ft long x 2 ft wide x 0.5 ft deep = 40 cubic feet = 1.48 cubic yards
That’s crusher run or 3/4” clean stone, compacted in 3-inch lifts. About 2 tons of material.
Backfill Gravel
Behind the wall and between the block and the retained soil, you need drainage gravel. This is 3/4” clean stone (no fines) that lets water pass through instead of building hydrostatic pressure against the wall.
The gravel zone should extend at least 12 inches behind the block, from base to within 6 inches of the top (where you cap it with soil to grow grass).
Volume for our 40-foot x 3-foot wall: 40 ft x 1 ft wide x 2.5 ft tall (3 ft wall minus 6” of topsoil cap) = 100 cubic feet = 3.7 cubic yards
About 5 tons. At $35/ton delivered, that’s $175 just in backfill stone.
Drainage Pipe
A perforated 4” drain pipe runs along the base of the wall, behind the first course, sitting in the gravel base. This carries water to a daylight outlet at the end of the wall or to a catch basin.
Length: wall length plus whatever extra run you need to reach a daylight point. A 40-foot wall might need 55 feet of pipe if the outlet is 15 feet past the end.
Use perforated pipe behind the wall (holes down, on top of the gravel base) and transition to solid pipe where it exits beyond the wall end. Add a filter fabric sock over the perforated section to keep fines out — or wrap the entire gravel zone in filter fabric. Either works.
When You Need Geogrid
Walls over 3-4 feet tall (varies by block manufacturer, soil conditions, and surcharge loads) need geogrid reinforcement. Geogrid is a plastic mesh that ties the wall face to the soil mass behind it, turning the whole thing into a gravity structure.
General guidelines (always check the block manufacturer’s engineering tables):
- Under 3 feet: No geogrid needed. Gravity alone holds it.
- 3-4 feet: One layer of geogrid, typically at the second or third course.
- 4-6 feet: Two layers, spaced evenly through the wall height.
- Over 6 feet: Engineering required. A stamped wall design from a geotechnical engineer. Three or more layers of geogrid, specific soil bearing requirements, and possibly a permit.
Geogrid extends back into the retained soil at least 60% of the wall height. A 4-foot wall needs geogrid extending 2.5 feet behind the block face. A 6-foot wall needs 3.6 feet. That means excavating a wider terrace behind the wall to lay the grid flat before backfilling over it.
Geogrid comes in rolls, typically 6 feet wide by 150 feet long. One roll covers a lot of linear wall footage. At $0.50-1.00 per sq ft, the material isn’t expensive. The extra excavation and labor is where the cost goes.
Curves and Corners
Inside curves use the same block count (the gaps between blocks at the back are just wider). Outside curves require cutting the block face at an angle so the fronts meet tight. Budget 15% waste for walls with curves, versus 5-10% for straight walls.
90-degree corners need corner blocks (if the manufacturer makes them) or blocks cut at 45 degrees on a wet saw. Not all block systems handle corners gracefully. Versa-Lok and Allan Block have corner units. Others require creative cutting.
Cost Estimate
For our 40-foot, 3-foot tall wall using standard blocks:
| Material | Quantity | Unit Price | Total |
|---|---|---|---|
| Wall blocks | 176 | $5 | $880 |
| Cap blocks | 28 | $7 | $196 |
| Base gravel (crusher run) | 2 tons | $35 | $70 |
| Backfill gravel (3/4” clean) | 5 tons | $35 | $175 |
| Drain pipe (4” perf) | 55 ft | $0.60/ft | $33 |
| Filter fabric | 1 roll | $45 | $45 |
| Adhesive | 2 tubes | $8 | $16 |
| Total materials | $1,415 |
Labor for a segmental retaining wall runs $10-25 per face square foot in most markets. Our 120 sq ft wall: $1,200-$3,000 installed. Materials are about half the total cost for contractor-built walls.
Running the Numbers
SiteCalc has a retaining wall calculator that takes wall length, height, and block size, then gives you block count, caps, base gravel, backfill gravel, and drain pipe. It accounts for waste and shows the formula behind each quantity. For a quick budget estimate, the cost-to-materials calculator works backward from a dollar amount to tell you how much wall it builds.